ALKYNYLCUBANES AS PRECURSORS OF RIGID-ROD MOLECULES AND ALKYNYLCYCLOOCTATETRAENES

We have developed new methodology for the synthesis of alkynylcubanes and have used these compounds to make rigid-rod molecules constructed of cubane and acetylene subunits. Terminal and substituted alkynylcubanes 7a, 7b, 8a, 8b, 12a, and 12b were synthesized by n-BuLi-promoted elimination of halogen from 1,1-dibromovinylcubanes 6a, 6b, and 11, followed by quenching with electrophiles. Systems with one or two acetylenic units between two cubanes were also prepared: dicubylacetylene (15) was obtained via reaction of the lithium ylide of (trimethylsilyl)diazomethane with dicubyl ketone (14); 1,4-dicubyl-1,3-butadiyne (16) was made by oxidative dimerization of ethynylcubane (7a). Dimerizations and cross-coupling reactions of various 1,4-diethynylcubanes afforded longer rods, e.g., 1,4-bis-((trimethylsilyl)ethynyl)cubyl- 1,3-butadiyne (18) and 1-(4-((trimethylsilyl)ethynyl)cubyl)-4-cubyl-1,3-butadiyne (21). Rh(I)-promoted ring opening of the cubane subunit(s) of these compounds into the corresponding tricyclooctadiene followed by thermal rearrangement to the cyclooctatetraene was used to convert 7a, 8a, and 12a into the mono- and disubstituted alkynylcyclooctatetraenes 22a, 22b, and 23 and to take 15 and 16 into the alkynyl-bridged cyclooctatetraenes 24a and 24b, respectively. X-ray crystallographic analysis of 12a, 15, 16, and 18 revealed interesting details about their structures.